In recent years, vast quantities of propylene polymers have been prepared via the well known Ziegler-Natta catalyst system. This catalyst consists essentially of the reaction product of titanium halide, typically TiCl.sub.4, with an alkyl aluminum compound, typically an alkyl aluminum sesquichloride. This is an extremely effective catalyst for preparing highly crystalline polymers and in its various forms, has become the industry standard.
More recently, much work has been done to develop new catalyst systems, one result of which has been the discovery of metallocene catalyst systems. Metallocene catalysts are defined as bridged bis dicyclopentadienyl or bis indenyl Group 4,5 or 6 transition metal dihalide derivatives. Specific metallocene catalysts known to be useful for producing polypropylene are discussed, inter alia, in EPA 485,820; 485,821; 485,822; 485,823; 518,092 and 519,237 and in U.S. Pat. Nos. 5,145,819 and 5,296,434. Other references that discuss the metallocene catalyzed process include EPA 351,932 and U.S. Pat. Nos. 5,055,438; 5,234,800; 5,272,016; 5,272,236 and 5,278,272. All of the cited documents are incorporated hereinto by reference.
The polymers resulting from metallocene catalysis are said to be of extremely uniform steric structure. Depending on the specific metallocene employed, the polymer can be of an a syndiotactic structure wherein the pendant methyl groups on the polymer chain are located on alternating carbon atoms and are alternately oriented above and below the plane of the chain. With another metallocene, the polymer can be of an isotactic structure wherein all of the pendant methyl groups are still located on alternate carbon atoms, but they are disposed predominantly on the same side of the chain.
In addition to the high degree of stereoregularity of the polymer chains, there is a very high degree of uniformity of molecular weight among the polymer chains, i.e., the molecular weight distribution is narrower than that found in conventional isotactic polypropylene. Thus, whereas a conventional polypropylene has a polydispersity (i.e. the ratio of weight average molecular weight to number average molecular weight) on the order of about 7, a metallocene catalyzed polypropylene prepared by the same manufacturer has a polydispersity on the order of about 3. Likewise, in the manufacture of copolymers or terpolymers, the metallocene catalyst leads to a more even distribution of the comonomer or comonomers throughout the product. Yet another good feature of the metallocene polymers is that they have a lower concentration of low molecular weight, xylene soluble materials.
The narrower molecular weight distribution of the metallocene catalyzed homopolypropylene and copolymer and the more even distribution of the comonomer in the copolymer lead to greater uniformity of the properties of the product whether it be the homopolymer or the coploymer. Thus, the metallocene catalyzed products exhibit a sharper melting point and a more uniform intrinsic viscosity throughout the product. These property improvements translate into improved flow properties and thus to improved processing during film extrusion and drawing. Processing is thus significantly facilitated.
However, in attempting to manufacture oriented packaging films with the metallocene catalyzed polypropylene in conventional processes and equipment, it has been found that the cast films exhibit very poor optical properties. Specifically, the films cast preparatory to drawing to effect orientation are extremely hazy, bordering on being translucent. In addition, they have a rough, sand paper-like surface. Drawn films prepared from these polymers are also found to exhibit a high degree of haze, making them unsuitable for high quality packaging applications.
It is an objective of this invention to provide a polymer composition containing metallocene catalyzed polypropylene that can be employed in the manufacture of polypropylene film. It is a further objective of the invention to provide superior polypropylene films comprising metallocene catalyzed polypropylene, which films exhibit a combination of improved properties as compared with polypropylene films heretofore known to the art.